There are many options for the front, as it's a common size. The rear size is difficult to find. Michelin makes an Ultra HD tube that fits, but it's too big and heavy to carry as a spare. Fine for keeping in the garage and using next time you change out your tires. Pirelli and Metzeler both make a normal duty tube in size listed as 140/80-18 that will fit fine (i've a metzeler in rear now). Not cheap, at around $30, compared to ~$15 for a common size. The Dunlop oem tube is back-ordered throughout the entire US.
I would not use front tube in the rear of this bike. It works in many dirt bikes, but they have a thinner rear. Besides, changing out a tube (especially the rear) is pain in the arse as it is - why make yourself do it twice? - once on the road, then again when you get home to put a proper rear tube in? Just carry a proper front and rear tube.